1. Field of the Invention
This invention relates to the field of modified diphenylmethane diisocyanates. More specifically it relates to storage stable, liquid, urethane-modified polyisocyanates which yield polyurethanes with superior mechanical properties.
2. Description of the Prior Art
The preparation of diphenylmethane diisocyanate compounds which are liquid at room temperature is known. According to the data in German Patent No. 16 18 380 (U.S. Pat. No. 3,644,457), this is achieved by reacting one mole 4,4,'- and/or 2,4'-diphenylmethane diisocyanate, abbreviated below as MDI, with from 0.1 to 0.3 mole tripropylene glycol and/or polyoxypropylene glycol having a molecular weight up to 700.
In GB Patent No. 1,369,334, the modification is performed in two reaction steps. Dipropylene glycol and polyoxypropylene glycol with a molecular weight under 2000 are used a the modifying agents.
German Published application OS No. 29 13 126 (U.S. Pat. No. 4,229,347) teaches MDI compounds in which from 10 to 35 percent by weight of the isocyanate groups are reacted with a mixture consisting of at least 3 alkylene glycols and whereby one of these glycols is a di-, tri-, or higher polypropylene glycol.
In German Published application OS No. 24 04 116 (GB Pat. No. 1,430,455), on the other hand, mixtures of a polyoxyethylene glycol or a polyoxyethylene glycol mixture with an average molecular weight less than 650 and at least one alkylene glycol with at least 3 carbon atoms are cited as the modifying agent.
German Published application OS No. 23 46 996 (GB Pat. No. 1,377,679) relates to MDI compounds in which from 10 to 35 percent by weight of the isocyanate groups were reacted with a standard commercial polyethylene glycol.
In order to prepare liquid polyisocyanate compounds, the reaction of mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates, subsequently abbreviated as crude MDI, are also described in addition to MDI, with glycols and/or polyoxyalkylene glycoIs.
According to EP Published application No. 10 850, the polyisocyanate compounds of this type consist of a mixture of crude MDI with an MDI modified with polyoxyalkylene polyols having a functionality from 2 to 3 based on polyoxypropylene polyol and, in some cases, on polyoxyethylene polyol, and a molecular weight from 750 to 3000.
A liquid crude MDI compound is attained, according to DE AS No. 27 37 338 (U.S. Pat. No. 4,055,548) by combining crude MDI with a polyoxyethylene glycol having an average molecular weight of from 200 to 600.
According to DE AS No. 26 24 526 (GB Pat. No. 1,550,325), crude MDI prepared in a special process and having 88 to 95 percent by weight MDI is reacted with polyoxypropylene glycol in the molecular weight range from 134 to 700.
DE Published application Nos. 25 13 796 (GB Pat. No. 1,444,192) and 25 13 793 (GB 1,450,660) relate to crude MDI compounds in which the crude MDI was modified with specified amounts of alkylene or polyoxyalkylene glycols.
The cited alkylene or polyoxyalkylene glycols do indeed produce a liquification of the 4,4'-, and 2,4'- MDI isomers, which melt at 42.degree. C. and 28.degree. C., respectively. The disadvantage, though, is that the polyisocyanate compounds exhibit crystalline precipitates when stored for relatively long periods of time at temperatures around -10.degree. C.
The preparation of flexible polyurethane foams using crude MDI compounds modified with urethane groups as the polyisocyanate component is also known.
According to EP Published application No. 22 617, this is done by using as the isocyanate component the reaction product of a di- to tri-functional polyoxypropylene polyoxyethylene polyol having a polymerized oxyethylene group content of at least 50 percent by weight with excess crude MDI. Particular disadvantages of the described foams are their low tensile and tear strengths.